1. Field of Invention
The present invention relates to a glass substrate-holding tool. The glass substrate-holding tool according to the present invention is useful for holding a glass substrate during the production of a reflective mask blank for EUV (Extreme Ultra Violet) lithography (hereinbelow, referred to as “EUV mask blank” in the specification) to be used for, e.g. the production of semiconductors, and during the production of a functional film-provided substrate for such an EUV mask blank.
Further, the present invention relates to a method for producing an EUV mask blank and a functional film-provided substrate for such an EUV mask blank by employing the glass substrate-holding tool according to the present invention.
2. Discussion of Background
In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for transferring a fine pattern on a silicon substrate or the like, which is required for forming an integrated circuit including such a fine pattern. However, the conventional photolithography method has reached near to the limit while semiconductor devices have had finer patterns at an accelerated pace. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than ArF lasers, has been considered as being promising as the exposure technique for 45 nm or below. In this specification, it should be noted that the phrase “EUV light” means a ray having a wavelength in a soft X ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of about 10 to 20 nm, in particular of about 13.5 nm±0.3 nm.
It is impossible to employ a conventional dioptric system like photolithography using visible light or ultraviolet light since EUV light is apt to be absorbed by any substances and since the refractive index of the substances is close to 1 at this wavelength. For this reason, a catoptric system, i.e., a combination of a reflective photomask and a mirror, is employed in EUV light lithography.
A mask blank is a stacked member to be used for fabrication of a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a substrate made of glass or the like has a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, formed thereon in this order. As the reflective layer, a Mo/Si multilayer reflective film is usually employed wherein molybdenum (Mo) layers as low refractive layers and silicon (Si) layers as high refractive layers are alternately stacked to increase a light reflectance when irradiating a layer surface with EUV light.
As the absorber layer, a material having a high absorption coefficient to EUV light, specifically, e.g. a material containing chromium (Cr) or tantalum (Ta) as the main component, is employed.
The multilayer reflective film and the absorber layer are formed on an optical surface of a glass substrate by, e.g. an ion beam sputtering method or a magnetron sputtering method. At the time of forming the multilayer reflective film and the absorber layer, the glass substrate is held by a holding tool. Although a mechanical chuck and an electrostatic chuck are used as a glass substrate-holding tool, it is preferred in terms of a reduction in dust generation that such an electrostatic chuck be used as the glass substrate-holding tool to catch and hold the glass substrate at the time of forming the multilayer reflective film and the absorber layer, in particular at the time of forming the multilayer reflective film.
The electrostatic chuck is a technique which has been heretofore used to catch and hold a silicon wafer in a process for producing semiconductor devices. The electrostatic chuck catches and holds a silicon wafer by an electrostatic attractive force which is generated by bringing a central portion of the silicon wafer into contact with a catching and holding surface having a planar shape in a circular shape, a rectangular shape or the like, and applying a voltage across electrode portions embedded in a dielectric layer forming the catching and holding surface.
For catching and holding a glass substrate, the electrostatic chuck is also used such that the entire portion of the glass substrate, including a central portion thereof, specifically the entire portion of a rear surface of the glass substrate (the entire portion of a rear surface of the glass substrate including a central portion thereof) opposite to the film deposition surface of the glass substrate with a multilayer reflective film and an absorber layer expected to be stacked thereon during the production of an EUV mask blank, is brought into contact with the catching and holding surface of the electrostatic chuck to carry out the catching and holding operation, for, e.g. reasons that the shape of the electrode portions is not complicated, that it is possible to provide a sufficient holding force, and that the caught and held glass substrate is prevented from inclining (see Patent Documents 1 to 9).
It has been heretofore considered that it is preferred to bring the entire portion of the glass substrate including the central portion thereof in contact with the catching and holding surface of the electrostatic chuck to carry out the holding operation since it is possible to make device design simple and since it is possible to reduce costs by utilizing an electrostatic chuck widely used for catching and holding a silicon wafer.
Hereinbelow, the surface of a glass substrate with a multilayer reflective film or an absorber layer expected to be stacked thereon during the production of an EUV mask blank is referred to as the “film deposition surface” of the glass substrate, and the rear surface of the glass substrate opposite to the film deposition surface is referred to as the “rear surface” in the specification.
However, when a glass substrate is held by bringing a central portion of the rear surface of the glass substrate into contact with the catching and holding surface of an electrostatic chuck, it is likely that foreign substances deposit on the central portion of the rear surface of the glass substrate, or that the central portion of the rear surface of the glass substrate is scratched. In the case of a glass substrate used for production of EUV mask blank, the central portion of the rear surface of the glass substrate is a portion that is also usually designated as a quality-guaranteed region for every device, such as an exposure system, and is required to be free from the deposition of foreign substances or the occurrence of scratches. From this point of view, the deposition of foreign substrates or the occurrence of scratches on the central portion of the rear surface could cause a serious problem.
It appears to be sufficient to bring an outer edge portion of the rear surface of a glass substrate except for the quality-guaranteed region into contact with the catching and holding surface of an electrostatic chuck in order to prevent the deposition of foreign substances or the occurrence of scratches on a central portion of the rear surface.
Patent Document 10 listed below discloses an exposure system which employs a non-contact type electrostatic chuck and a contact type electrostatic chuck to hold a reflective mask, although having no purpose of preventing foreign substances from adhering to such a central portion or such a central portion from being scratched. In this exposure system, a reflective mask has a central portion of its rear surface placed in a non-contact state with the non-contact type electrostatic chuck by a gap and is held by the contact type electrostatic chuck at three points on a peripheral portion of the rear surface. It is regarded that the gap between the central portion of the rear surface of the reflective mask and the non-contact type electrostatic chuck is preferably at most 20 μm, more preferably at most 10 μm, further preferably at most 5 μm.
However, when the glass substrate is held by bringing the peripheral portion of the rear surface into contact with the catching and holding surface of the electrostatic chuck, the surface area of a portion of the glass substrate in contact with the catching and holding surface of the electrostatic chuck (hereinbelow also referred to as “the caught and held portion” of a glass substrate in the specification) is reduced. Accordingly, when an attempt is made to provide a catching and holding force enough to hold the glass substrate, the pressure per unit area applied to the caught and held portion is increased, which is likely to cause a problem of occurrence of scratches on the caught and held portion or of occurrence of foreign substances caused by such scratches. Further, a large amount of charged foreign substances are likely to be attracted by an electrostatic field generated at the caught and held portion.
Since the caught and held portion is present on the peripheral portion of the rear surface, the effect caused by the occurrence of scratches or foreign substances is minor than a case where a similar problem is caused on the quality-guaranteed region of the rear surface. However, it is likely that foreign substances formed at the caught and held portion or foreign substances attracted to the caught and held portion are partly transferred to the quality-guaranteed region of the rear surface. When the caught and held portion is scratched, it is likely that the force required for holding the glass substrate is reduced in a post-process for a mask blank produced on the glass substrate. Specifically, an electrostatic chuck is used as the glass substrate-holding tool to catch and hold a glass substrate in a mask patterning process for fabricating a reflective mask from an EUV mask blank, or in handling of a reflective mask for exposure in EUV lithography. If the caught and held portion is scratched to form a step, it is likely that the flatness of the caught and held portion is degraded to reduce the catching and holding force of the electrostatic chuck.
On the other hand, when an attempt is made to reduce the catching and holding force of an electrostatic chuck in order to prevent the caught and held portion from being scratched or a large amount of foreign substances from being attracted to the caught and held portion, the catching and holding force applied to a glass substrate become insufficient. During the production of an EUV mask blank, a multilayer reflective film or an absorber layer is normally deposited with a glass substrate being rotated, or with a glass substrate being longitudinally placed so as to have the film deposition surface vertically oriented according to the structure of a film deposition system in some cases. If the catching and holding force applied to the glass substrate become insufficient in such cases, the glass substrate is likely to be displaced or come off.
Patent Document 10 exemplifies a vacuum absorption device, an electromagnetic attraction device and a mechanical holding device in addition to an electrostatic attraction device (i.e. a contact type electrostatic chuck) as a device for holding a reflective mask at a peripheral portion of its rear surface. However, the use of an electromagnetic attraction device has a similar problem to a case where a contact type electrostatic chuck is used.
This is also applicable to the use of a vacuum absorption device or a mechanical holding device since the surface area of a portion of a glass substrate in contact with such a device is small. Under this circumstance, when an attempt is made to provide a holding force enough to hold the glass substrate, the pressure per unit area applied to the held portion of the glass substrate is increased, which is likely to cause a problem of occurrence of scratches on the caught and held portion or of occurrence of foreign substances caused by such scratches as in the use of an electrostatic attraction device. Such a vacuum absorption device cannot fulfill its function in a highly vacuum state where it is impossible to ensure a sufficient absorbing force, i.e. a state where the relationship of (force required for absorption)>(ultimate vacuum pressure)×(contact area) is established. In other words, it is practically impossible to hold a silicon wafer or a glass substrate by vacuum absorption in a vacuum state having at most 1 Pa, which is generally regarded as being a high vacuum state.
Further, Patent Document 10 recites that the gap between a central portion of the rear surface of a reflective mask and a non-contact electrostatic chuck is preferably at most 20 μm, more preferably at most 10 μm, further more preferably at most 5 μm. However, in a case where the gap between a central portion of the rear surface of a reflective mask and a non-contact electrostatic chuck is such a narrow gap, if foreign substances attracted by an electrostatic attractive force from the electrostatic chuck are caught in the gap, a glass substrate and the electrostatic chuck are both scratched, which fails to solve a problem of preventing scratches in spite of providing such a gap.
Furthermore, if foreign substances are caught in the gap between such a central portion of the rear surface of the glass substrate and the non-contact electrostatic chuck, the glass substrate is likely to be deformed. If the glass substrate is deformed, the glass substrate is likely to be further deformed by a film stress caused in a multilayer film or an absorber layer.
In the exposure system disclosed in Patent Document 10, a reflective mask is deformed since a central portion of the rear surface of the reflective mask and a non-contact electrostatic chuck are placed in a non-contact state so as to have a gap therebetween. The deformation of a glass substrate during the production of an EUV mask blank is not preferred as described above.